This invention relates generally to apparatus and methods for controlling altitude and more particular, but not by way of limitation, to apparatus and methods for controlling the altitude of submersible vehicles.
In a submarine environment, for example, it is often necessary to maintain a submersible vehicle working along the sea floor at a selectable altitude so that the vehicle can properly perform its work. This altitude must be maintained to insure the safety of the submersible vehicle and to insure the quality of survey data, for example, which are to be obtained by equipment on the vehicle.
It is known that the depth of a submersible vehicle below the surface of a body of liquid in which the vehicle is operating can be maintained by using feedback from a hydrostatic pressure transducer or other suitable depth measuring device. It is likewise known that a submersible vehicle can be controlled at a selected altitude above the sea floor by using feedback from an echo-sounding sonar device or other suitable altitude measuring device. However, when using only one or the other of such known devices, it is difficult to maintain suitable control over the vehicle because of the deficiencies inherent in each of these devices.
For example, altitude measuring devices become problematical as the altitude of the submersible vehicle above the sea floor increases because the sampling rate for altitude measurements must decrease due to the greater travel time of the acoustic signals. Additionally, errors in determining the exact time at which the echoes are received from the sea floor can arise. Therefore, as the altitude of the submersible vehicle above the sea floor increases, altitude measurement devices are not as desirable as depth measuring devices which offer greater resolution and accuracy at such increased altitudes. On the other hand, when the submersible vehicle is to be operated relatively close to the sea floor, such as in performing bottom-surveying work where the submersible vehicle moves horizontally along and close to the sea floor, control by means of altitude measuring devices is more desirable because it is distance above the sea floor (i.e., altitude), rather than distance below the surface (i.e., depth), which is more critical to the safety and quality of work of the submersible vehicle.
Therefore, there is a need for an apparatus and a method which give proper altitude control, but which also give stability of operation so that the submersible vehicle is not continually rising and sinking with each change in the terrain of the sea floor. Thus, there is the need for an apparatus and a method which include the desirable features of both an altitude measuring device and a depth control device.
Although it is desirable to combine the altitude measuring device features with those of the depth measuring device as aforementioned, the use of a depth measuring device can at times be problematical because depth measurement errors can arise due to wave action or to changes in the specific gravity of the water in which the vehicle is to be operated. Therefore, there is also the need for an apparatus and method which combines an altitude measuring device with a vertical acceleration detection device to overcome the shortcomings in the combined altitude measuring device and depth measuring device embodiment.
More generally, there is the need for an apparatus and a method which control the altitude of an object over a relatively long period of time in response to a signal related to the detected altitude, but which control the altitude of the object over a shorter period of time in response to a vertical velocity signal derived from a depth signal or an acceleration signal.
It is to be noted that the broad concept of using more than one ambient condition, such as altitude or depth or acceleration, in an attempt to control altitude is known. See, for example, U.S. Pat. No. 3,958,219 to Bateman et al., U.S. Pat. No. 4,215,334 to Bateman, and U.S. Pat. No. 4,012,713 to Greene et al. However, we are unaware of any apparatus or method of the type disclosed and claimed herein.